Syringe with needle guard injection device

ABSTRACT

The present invention provides for an injection device including a syringe with a proximal end, a distal end, a needle extending from the distal end, and a plunger inserted into the proximal end. A guard is slidable on the syringe, has a proximal end, a distal end, and is biased from a first position wherein the needle is exposed towards a second position wherein the guard covers the needle. The injection device includes first and second sets of cooperating detents for retaining the guard and syringe in the first and second positions, respectively. A latch member extends proximally from the guard that is engageable by the plunger as the plunger is depressed to release the first set of cooperating detents. Upon release, a spring element biases the guard towards the second position until the second set of cooperating detents engage and secure the guard in the second position.

RELATE BACK INFORMATION

This application is a continuation of Ser. No. 10/113,537, filed Mar.27, 2002, now U.S. Pat. No. 6,976,976.

FIELD OF THE INVENTION

The present invention relates generally to injection devices foradministering therapeutic agents to patients, particularly to safetydevices for syringes, and more particularly to syringe devices thatinclude a needle guard slidably coupled to a syringe for covering aneedle of the syringe after use.

BACKGROUND

Medication is often dispensed using a medicine cartridge, such as asyringe, having a barrel with a needle extending from one end and aplunger slidably inserted into the other end. Such cartridges are oftenreferred to as “pre-filled syringes” because they may contain a specificdosage or volume of medication when they are initially provided, ascompared to conventional syringes that are furnished empty and filled bythe user before making an injection.

Alternatively, a medicine cartridge may be used, such as an ampoule orvial, that includes a penetrable seal instead of a needle on one end ofthe barrel, and/or a piston rather than a plunger on the other end. Suchmedicine cartridges are generally inserted into an adapter that includesa hollow body adapted to hold the cartridge, a plunger to engage andmove the piston in the cartridge, and/or a double-ended needle topenetrate the seal and communicate with the interior of the barrel.

Because of the risk of communicable diseases, a number of syringes andadapters have been developed that are intended to prevent accidentalneedle sticks and/or inadvertent reuse of a syringe. Retractable needledevices have been suggested for this purpose that include a cartridgethat allows a needle of the cartridge to be withdrawn into the barrelafter medication is dispensed from it. For example, U.S. Pat. No.4,973,316 issued to Dysarz discloses a syringe including a barrel havinga needle assembly that is slidable within the barrel between an exposedposition such that a needle on the assembly extends from the barrel anda retracted position wherein the needle assembly is withdrawn into thebarrel. The needle assembly is initially locked in the exposed position,but may be disengaged upon depression of the plunger, whereupon a springbiases the assembly towards the retracted position, thereby withdrawingthe needle into the barrel.

Alternatively, syringe holders have been suggested that include a bodywithin which a conventional syringe or cartridge may be received, and ashield that is manually slidable with respect to the body to cover theneedle. For example, U.S. Pat. No. 6,030,366, issued to Mitchell, whichis assigned to the assignee of the present application, discloses aself-shielding guard that includes a body having an open proximal endfor inserting a syringe into a cavity within the body, and a distal endwith an opening through which a needle on the syringe may extend oncereceived in the body. A shield is slidable over the body betweenretracted and extended positions to expose and cover the needle,respectively. With the shield in the retracted position and the needleexposed, an injection may be made, and then the shield may be manuallyadvanced to the extended position. In the extended position, cooperatingdetents and detent pockets on the body and shield substantiallypermanently lock together, thereby preventing reuse of the needle,reducing the risk of accidental needle sticks, and/or facilitatingdisposal of the syringe.

As an alternative to requiring manual extension of a shield to cover aneedle, spring-loaded devices have also been suggested. These devicesoften include a body and slidable shield, similar to the manual devicesdescribed above, but also may include a spring element to bias theshield to advance and cover the needle. An actuator, such as a button orlever, may be activated by the user to release the shield, therebyallowing the spring element to advance the shield to cover the needle.For example, U.S. Pat. No. 5,695,475 issued to Best, Jr. et al. and U.S.Pat. No. 4,923,447 issued to Morgan disclose spring-loaded syringedevices that include inner and outer sliding sleeves that include abutton slidable in a longitudinal slot to selectively expose and cover aneedle on the devices. A spring in the devices biases one of the sleevesto extend and cover the needle, but this bias may be manually overcometo expose the needle. Thus, these devices may not lock the extendingsleeve in a covered position, and therefore may risk accidental needleexposure and/or reuse of the needle. In addition, although these devicesare spring-driven, their shields may not extend unless they are manuallyactivated by using a button, and therefore are not truly “passive,” butrequire an affirmative decision by a user to activate their safetyfeature.

Accordingly, an injection device that includes a needle shield forcovering a needle after use would be useful.

SUMMARY OF THE INVENTION

The present invention is directed to injection devices incorporatingboth a syringe, such as a pre-filled syringe, or a medicine cartridge,and a guard that covers a needle of the syringe after a medication inthe syringe is injected into a patient. The present invention is alsodirected to methods for making and using such injection devices.

In accordance with one aspect of the present invention, an injectiondevice is provided that includes a syringe having a proximal end, adistal end, and a plunger extending from the proximal end. The injectiondevice also includes a guard having a proximal end, a distal end, and acavity therebetween. The guard is slidably mounted to the syringe andincludes a latch member that extends proximally from the guard. A needlemay extend from the distal end of the syringe. The guard is biased froma first or retracted position wherein the needle of the syringe isexposed, towards a second or extended position wherein the guard coversthe needle. The guard may be biased by a spring element located in thedistal portion of the guard and/or coupled to the syringe.

First cooperating detents on the syringe and the guard act to retain theguard in the first position. The first cooperating detents may include aledge on the proximal end of the syringe and a catch on the latch memberextending from the guard. Here, the ledge and detent engage one anotherto retain the guard in the first position. As the plunger is advancedwithin the syringe, the plunger may engage the latch member and releasethe first cooperating detents, whereupon the guard may slide towards thesecond position. Second cooperating detents may be provided on thesyringe and guard to retain the guard in the second position. The secondcooperating detents may include a detent on the guard and acorresponding detent pocket on the syringe.

Optionally, the guard may also include a window or slot that extendsaxially along the guard. When the slot is present on the guard, thesyringe may include a stop tab configured to travel within the slot. Thestop tab and slot may be configured to limit the proximal and distalmovement of the syringe relative to the guard. For example, the stop tabmay abut the distal edge of the slot or window when the guard is in thefirst position to prevent further distal movement of the syringe (orproximal movement of the guard). Similarly, the stop tab may abut theproximal edge of the slot when the guard is in the second position toprevent further proximal movement of the syringe (or distal movement ofthe guard). In addition, the guard may also include a set of fingerflanges, grips, or protrusions usable for controlling the speed ofadvancement or sliding of the guard between the first and secondpositions.

In accordance with another aspect of the present invention, an injectiondevice includes a guard having a proximal end and a distal end. Theguard may be configured for slidably engaging a syringe, and may bebiased between a first or retracted position and a second or extendedposition relative to the syringe, e.g., by a spring element. Theinjection device may also include a syringe having a proximal end, adistal end including a needle, and a plunger that extends from theproximal end. The needle may be coupled to the distal end of thesyringe, i.e., molded to the distal end or a hub may be provided on thedistal end of the syringe to which a needle may be attached, e.g., bythreads or a luer lock. A latch member extends proximally from the guardthat is detachably coupled with the syringe, thereby retaining the guardin the first position. The latch member is engageable by the plunger asthe plunger is depressed for decoupling the latch member from thesyringe. When the latch member is decoupled, the guard may be advancedfrom the first position towards the second position.

In a preferred embodiment, the latch member may include an intermediateportion that is engageable with a flange on the syringe, and a proximalportion that is engageable with the plunger as the plunger is advancedinto the syringe. When the plunger engages the proximal portion of thelatch member, the intermediate portion of the latch member may bedeflected and released from the flange of the syringe. Additionally,cooperating detents on the guard and the syringe may secure the guardonce the guard is advanced to the second position.

In accordance with yet another aspect of the present invention, a methodis provided for delivering a therapeutic agent using an injectiondevice. The injection device may include a guard having a proximal end,a distal end, and a latch member extending from the proximal end. Theinjection device may also include a syringe received in the guardincluding a needle extending from a distal end thereof, and a plungerextending from a proximal end thereof. The guard may be biased from afirst position, wherein the needle is exposed, towards a secondposition, wherein the needle is covered by the guard.

Initially, the latch member may retain the guard in the first position.The needle may be inserted into a patient's skin while the user holds aportion of the injection device. A distal force may be applied to theplunger to inject the medication into the patient until the plungercontacts the latch member. The plunger may be depressed further todeflect the latch member, whereupon the guard may be released from thefirst position. The plunger may then be depressed yet further tocomplete delivery of the therapeutic agent. The user may then withdrawthe needle from the patient's skin, and release the plunger, whereuponthe syringe may retract proximally with respect to the guard until theneedle is disposed within the guard. When the guard attains the secondposition, cooperating detents on the guard and the syringe may engageone another to lock the guard in the second position and/or preventsubsequent distal movement of the syringe relative to the guard.

Other objects and features of the present invention will become apparentfrom consideration of the following description taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are side views of a preferred embodiment of an injectiondevice, in accordance with the present invention.

FIGS. 2A and 2B are side views of a syringe for use in the injectiondevice of FIGS. 1A and 1B.

FIG. 2C is a perspective view of the syringe of FIGS. 2A and 2B.

FIGS. 3A and 3B are side views of a guard for use in the injectiondevice of FIGS. 1A and 1B.

FIG. 3C is a perspective views of the guard of FIGS. 3A and 3B.

FIGS. 3D and 3E are end views of the guard of FIGS. 3A and 3B, takenfrom distal and proximal ends of the guard, respectively.

FIG. 4A is a cross-sectional side view of the injection device of FIGS.1A and 1B, with the guard in a retracted position relative to thesyringe.

FIG. 4B is a detail of the proximal end of the injection device of FIG.4A.

FIG. 4C is a detail of the distal end of the injection device of FIG.4A.

FIGS. 5A-5C illustrate an alternative spring element for use with aninjection device, in accordance with the present invention.

FIGS. 6A-6B illustrate another alternative spring element suitable foruse with an injection device, in accordance with the present invention.

FIGS. 7A-7B illustrate yet another spring element that may be used withan injection device, in accordance with the present invention.

FIGS. 8A-8C illustrate one method of using an injection device of thepresent invention to perform an injection.

FIGS. 9A-9C illustrate another method of using an injection device ofthe present invention to perform an injection.

DETAILED DESCRIPTION

Turning to FIGS. 1A-4C, a preferred embodiment of an injection device500 is shown for administering a medication, e.g., a vaccine, drug, orother therapeutic or diagnostic agent, to a patient. The device 500includes a syringe 550 including a needle 556, and a guard 502 slidablyattached to the syringe 550. The guard 502 is preferably biased from afirst position wherein the needle 556 is exposed (shown in FIG. 1A)towards a second position wherein the guard 502 covers the needle 556(shown in FIG. 1B).

As best seen in FIGS. 2A-2C, the syringe 550 includes a barrel 552, adistal end or hub 554 from which the needle 556 extends, and a plunger553 inserted into a proximal end 522 of the barrel 552. In oneembodiment, the needle 556 is attached to the hub 554 duringmanufacturing, and the device 500 is provided to a user with the needle556 pre-attached. In another embodiment, the hub 554 includes aconnector, e.g., a thread or luer lock adaptor. Thus, the device 500 maybe provided to a user without a pre-attached needle 556, and the usermay attach a desired needle 556 to the hub 554 before use.

Although a substantially square cross-section is shown, the barrel 552may have a substantially cylindrical cross-section, rectangularcross-section, or other suitably shaped cross-section. As illustrated,one or more stop tabs 538 may be provided on the barrel 552. The stoptabs 538 extend transversely outwardly, e.g., on opposite sides of thebarrel 552. The plunger 553 may include a thumb pad 551 on its proximalend to move the plunger 553 axially within the barrel 552. The thumb pad551 may have a diameter or other cross-section that is larger than adiameter of the barrel 552 of the syringe 550. In addition, oralternatively, the plunger 553 may include an intermediate tab (notshown) located distally to the thumb pad 551.

In the illustrated embodiment, one or more sets of ledges 540 may beprovided, e.g., molded, on or near the proximal end 522 and on one ormore side walls of the syringe 550. Preferably, each ledge 540 has asloping distal edge 540 a and a substantially blunt proximal edge 540 bdefining a detent pocket adjacent the proximal edge 540 b.

In addition, the syringe 550 may include one or more sets of detentpockets 542, 543 adjacent the ledges 540. A proximal set of detentpockets 543 may have substantially blunt proximal edge 543 b and slopingor substantially blunt distal edges 543 a. A distal set of detentpockets 542 may be provided at a location on the syringe 550 distal tothe ledges 540 and/or adjacent the distal end 554 of the syringe 550.Similar to the proximal detent pockets 543, the distal detent pockets542 may have substantially blunt proximal edges 542 b and have slopingor substantially blunt distal edges 542 a.

Turning to FIGS. 3A-3E, the guard 502 is a tubular member having aninner cross-section configured to slidably fit on the syringe 550. Forexample, the inner cross-section of the guard 502 may be substantiallyrectangular, square, or cylindrical in cross-section, depending upon thecross-section of the syringe 550. The guard 502 includes one or moreside walls 561 a, 561 b, an open proximal end 562, and an open distalend 563 defining a passage therebetween. Assembly tabs 572 with slopingor ramped interior surfaces 573 may be molded into and extend proximallyfrom the side walls 561 a. Optionally, at least one side wall 561 a,e.g., opposing walls 561 a, may include an elongate opening or slot 564therethrough for receiving the stop tabs 538 and/or for facilitatingobservation of the syringe 550 received in the guard 502. A pair offinger grips 515 may be located on the guard 502, e.g., on opposite sidewalls 561 b.

One or more latch members or fingers 574 extend proximally from theguard 502, e.g., molded to the side wall(s) 561 b. Each latch member 574includes an inwardly disposed catch or tab 576 located on anintermediate portion of the latch member 574 between the assembly tab572 and a proximal tip 578 of the latch member 574. The catches 576 andthe ledges 540 may provide a set of cooperating detents for retainingthe guard 502 in the first position. Alternatively, in embodiments wherethe syringe 550 lacks the ledges 540 (not shown), the catches 576 mayengage with a flange on the proximal end 522 of the syringe 550, or mayengage the proximal end 522 itself, thereby defining a proximal set ofcooperating detents that retain the guard 502 in the first position.Preferably, the latch members 574 are substantially flexible orsemi-rigid, and may be curved such that the intermediate portions aredisposed radially outwardly further than the proximal tips 578 when thelatch members 574 are in a relaxed, unstressed state. Thus, if theproximal tips 578 are directed distally, the latch members 574 maypartially bend or bow, thereby directing the intermediate portions, andconsequently the catches 576, transversely outwardly.

A plurality of detent arms 570 and a plurality of detents 571 areprovided on the guard 502, preferably molded directly to the side walls561 b. The detents 571 preferably have shapes correspondingsubstantially to the shapes of the distal detent pockets 542 in thesyringe 550. For example, distal edges 571 a of the detents 571 may beramped to facilitate slidable engagement with the proximal detentpockets 543 in the syringe 550. Proximal edges 571 b of the detents 571are substantially blunt for positively engaging the proximal edges 542 bof the distal detent pockets 542 and locking the guard 502 in the secondposition. Together, the detents 571 and distal detent pockets 542 atleast partially define a second set of cooperating detents for securingthe guard 502 in the second position.

As best seen in FIGS. 1B, 4A, and 4C, the injection device 500 mayinclude a spring element 504 for biasing the guard 502 towards thesecond position. The syringe 550 may include a substantially bluntdistal surface 525 that is proximal from the hub 554, and the guard 502may include a substantially blunt inner collar 567. The spring 502 maybe positioned concentrically within the guard 502 such that respectiveends of the spring 504 engage the distal surface 525 and the innercollar 567. Thus, the guard 502 and syringe 550 together at leastpartially define an annular space within which the compression spring504 may be compressed when the guard 502 is in the first position.Alternatively, other configurations may be provided for the springelement 504, such as those shown in FIGS. 5A-7B, and described furtherbelow.

The injection device 500 is pre-assembled such that the guard 502 isdisposed in the first position, as illustrated in FIG. 1A. In the firstposition, the catches 576 on the latch members 574 may engage the ledges540 of the syringe 550 to retain the guard 502 in the first position.Specifically, the catches 576 on the latch members 574 engage theproximal edges 540 b of the ledges 540 on the syringe 550, as seen inFIG. 1A, thereby retaining the guard 502 in the first position. Inembodiments of the injection device 500 that lack the ledges 540, thecatches 576 may directly engage the proximal end 522 of the syringe 550or a flange (not shown) on the syringe 550. Optionally, in the firstposition, stop tabs 538 on the syringe 550 may abut the distal edge 568of slots or windows 564 in the guard 502, as shown in FIG. 1A.Additional information on assembling the guard 502 and the syringe 550,may be found in co-pending application Ser. No. 09/724,657, filed Nov.28, 2000, the disclosure of which is expressly incorporated herein byreference.

Once assembled with the guard 502 in the first position, the injectiondevice 500 may be used to inject medication from the barrel 552 into apatient (not shown). A needle cover (not shown) may be removed from theneedle 556, and the needle 556 inserted into the patient. Medication maythen be delivered by directing the plunger 553 distally with the user'sthumb until the thumb pad 551 contacts the tips 578 of the latch members574. While using the injection device 500, the user preferably places afinger on each finger grip 515 and the thumb on the thumb pad 551. Asthe plunger 553 is depressed further, the thumb pad 551 may cause thelatch members 574 to compress axially, thereby deflecting theintermediate portions radially outwardly until the catches 576 aredisengaged from the ledges 540 on the syringe 550.

With the ledges 540 and catches 576 disengaged, the spring element 504automatically biases the guard 502 distally towards the second position,i.e., to advance the distal end 563 of the guard 502 over the needle556. The syringe 550 may be maintained substantially stationary relativeto the patient's skin, and the plunger 553 may be depressed further, ifnecessary, to complete the injection. When the catches 576 aredisengaged, the guard 502 may advance until it contacts the patient'sskin, and the needle 556 may be withdrawn from the patient, allowing theguard 502 to extend entirely over the needle 556. As the guard 502advances, the latch members 574 may slide distally along the syringe550.

When the needle 556 is withdrawn from the patient, the guard 502 mayadvance fully to the second position such that the guard 502 passes overand substantially covers the needle 556. In the second position, shownin FIG. 4A, the detents 571 of the guard 502 enter and substantiallyengage the distal detent pockets 542 of the syringe 550, therebypreventing subsequent proximal movement of the guard 502. Stop tabs 538on the syringe may also abut the proximal edges 566 of the slots 564,thereby preventing further distal movement of the guard 502. Thus, theguard 502 may be substantially permanently locked in the second orextended position, thereby preventing reuse of the injection device 500and/or facilitating safe disposal of the injection device 500.

In the second position, the spring element 504 may remain slightlycompressed to provide a distal force that may ensure that the guard 502is fully advanced and the detents 571 are received in the distal detentpockets 542.

In further alternatives, additional elements may be provided on thesyringe 550 and/or guard 502 to control the speed of advancement of theguard 502 from its initial activation in the retracted position, butallow free advancement as the guard 502 approaches the extendedposition. For example, a frictional material (not shown) may be appliedto one or more contact surfaces between the syringe 550 and guard 502that may increase the frictional resistance in the retracted position.An adhesive tape or viscous material (not shown) may be provided thatpeels off of, is cut or torn, or otherwise separates from the syringe550 and/or guard 502 after partial advancement of the guard 502. One ormore “speed bumps” or similar protruding structures (not shown) may bemolded into or otherwise provided on the syringe 550 and/or guard 502that may be contacted when the guard 502 is in or close to the retractedposition. A feature on one of the syringe 550 or guard 502 may travelthrough a corresponding tortuous feature on the other of the syringe 550or guard 502 to slow the guard 502 down initially. In a furtheralternative, a spring or cushioned pad (not shown) may be provided onthe distal end of the guard 502 that may soften the force with which theguard 502 contacts the patient if the guard 502 is allowed to advance.

In the embodiment described above, the spring element 504 is a helicalcompression spring. Alternatively, one or more springs (not shown) maybe provided, e.g., in one or more corners of the guard 502. For example,springs may be provided in diagonally opposite corners of the guard 502in order to minimize the risk of subjecting the syringe 550 to torque asit advances from the retracted towards the extended position as mayoccur if only a single off-center spring is used. In anotheralternative, one or more helical extension springs 504 a may be providedthat have respective ends that are secured to the syringe 550 and theguard 502, as is shown schematically in FIGS. 5A-5C. The extensionspring 504 a may be provided between the guard 502 and the syringe 550.The extension spring 504 a may be under tension when the guard 502 isdisposed in the retracted position, thereby biasing the guard 502towards the extended position.

Other alternative spring elements may also be provided that may becoupled to the guard 502 and syringe 550. FIGS. 6A and 6B illustrate anembodiment of the present invention having a coil spring element 504 bthat has one end mounted around a hub 584 a on the guard 502 (attachmentpoint of hub 584 a to the guard 502 not shown) and the other end securedto the syringe 550. The coil spring element 504 b may unwind as theguard 502 is retracted to the retracted position, as in FIG. 6A, but isbiased to wind back around the hub 584 a to advance the guard 502towards the extended position, which is shown in FIG. 6B. In anotheralternative, illustrated in FIGS. 7A and 7B, a leaf spring element 504 cmay be provided that has ends fixed to one of the guard 502 and thesyringe 550 for distally biasing a hub 584 b coupled to the other of thesyringe 550 and the guard 502 (attached point of hub 584 b to body 502not shown). The guard 502 may be retracted, thereby deflecting the leafspring element 504 c, as in FIG. 7A, but when the guard 502 is released,the hub 584 b and consequently the guard 502 may be advanced to theextended position as the leaf spring element 504 c attempts to return toits relaxed, undeflected state, as in FIG. 7B.

In addition, if desired, any of the spring elements 504-504 c may have anonlinear spring rate, preferably having a spring rate when the guard502 is disposed in the retracted position that is substantially more orless than its spring rate when the guard 502 approaches the extendedposition. A nonlinear spring rate may be useful to provide an injectiondevice 500 that is “softer” upon activation, i.e., such that the springelement (504-504 c) may apply a substantially lower force to the guard502 when it is initially released from the retracted position than whenit approaches the final extended position. For example, the thickness ofthe spring material may be varied along its length, different materialsmay be used for different portions of the spring, and the like. This“softer” spring may minimize the force with which the guard 502 contactsa patient being injecting using the injection device 500, and therebyreduce the discomfort or anxiety that may be experienced by the patient.For example, the guard 502 may be activated with a softer force, suchthat the guard 502 more gently contacts the patient, but when the needleis withdrawn from the patient, a stronger force may be applied to theguard 502 to ensure that it advances completely to the extendedposition.

Turning to FIGS. 8A-8C, another method is shown for performing aninjection using an injection device 500. Unlike the previousembodiments, which generally discuss a guard advancing over a syringe,this method involves holding the guard 502 such that the syringe 550 maybe automatically and/or controllably directed proximally to retract theneedle 556 into the guard 502. The guard 502 and syringe 550 areslidably attached to each other, similar to the previous embodiment. Theguard 502 includes a spring element 504, similar to the previousembodiments, for biasing the syringe 550 towards a second positionwherein the guard 502 substantially covers the needle 556.

As best seen in FIG. 8A, a needle cover (not shown) may be removed fromthe needle 556, and the needle 556 may be inserted into a patient's skin700. A user may grasp finger grips 515 or otherwise grip an outersurface of the guard 502. Thus, the guard 502 may be grasped to hold theinjection device 500 substantially stationary with respect to the skin700 during the injection.

To deliver medication from the syringe 550, the user may grip the fingergrips 515 and apply a distal force to the plunger 553, preferably untilthumb pad 551 engages latch members 574 extending from the guard 502. Asthe plunger 553 is depressed further, the thumb pad 551 may cause thelatch members 574 to compress axially and thereby deflect radiallyoutwardly until the catches 576 (not shown in FIGS. 8A-8C) aredisengaged from the ledges 540 (also not shown) of the syringe 550, orare disengaged from the proximal end 522 if the syringe 550 does notinclude the ledges 540. With the catches 576 disengaged, the springelement 504 may automatically bias the syringe 550 towards the second orretracted position, shown in FIG. 8C.

Once the ledges 540 or the proximal end 522 are disengaged from thecatches 576, the user may gradually remove the distal force applied tothe plunger 553, allowing a controlled retraction of the syringe 550 tothe retracted position. As the syringe 550 retracts to the retractedposition, the guard 502 may remain substantially stationary with respectto the patient's skin 700, and the needle 556 may be withdrawn from thepatient and into the guard 502, as shown in FIGS. 8B and 8C. Once in thefully retracted position, cooperating detents 571, 542 on the guard 502and syringe 550 engage one another to substantially and permanently lockthe guard 502 over the needle 556. Thus, the user may hold the fingergrips 515 to position the injection device 500 a predetermined distanceway from the patient's skin 700, thereby avoiding any sudden contactbetween the guard 502 and the patient's skin 700 when the ledges 540, orproximal end 522, of the syringe 550 and the catches 576 are disengaged.

Alternatively, as shown in FIGS. 9A-9C, the user may hold the injectiondevice 500 using finger grips 515, insert the needle 556 into thepatient's skin 700, and then inject medication by applying a distalforce to the plunger 553 until the thumb pad 551 engages a latch member574 extending from the guard 502. As described above, further depressionof the plunger 553 deflects the latch member 574 to disengage thecatches 576, and release the guard 502. The guard 502 may thenautomatically advance into contact with the patient's skin 700, as shownin FIG. 9B. The plunger 553 may be further advanced to complete theinjection. Preferably, because the spring element 504 is not fullyexpanded, the syringe 550 remains biased to retract with respect to theguard 502 towards the retracted position. At this point, the user maygradually remove the distal force applied to the plunger 553, allowingcontrolled retraction of the syringe 550 to the retracted position, asshown in FIG. 9C. Once the syringe 550 retracts to the retractedposition, the cooperating detents 571, 542 may engage one another tosubstantially permanently lock the guard 502 over the needle 556,similar to the embodiment discussed above.

While the invention is susceptible to various modifications, andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formsor methods disclosed, but to the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the appended claims.

1. An injection device, comprising: a syringe comprising a proximal end, a distal end, and a plunger extending from the proximal end, the syringe having proximal and distal pockets; a guard slidably attached to the syringe and having a proximal end and a distal end, the guard being biased from a first position wherein the distal end of the syringe is exposed towards a second position wherein the guard covers the distal end of the syringe; cooperating detents on the guard for engaging the syringe pockets for retaining the guard in the first position, and for retaining the guard in the second position; and a latch member extending proximally from the guard, the latch member being engageable by the plunger as the plunger is advanced into the syringe for releasing first cooperating detents whereupon the guard may slide toward the second position.
 2. The device of claim 1, further comprising a needle extending from the distal end of the syringe.
 3. The device of claim 1, further comprising a spring coupled to the guard and the syringe, the spring biasing the guard toward the second position.
 4. The injection device of claim 1, wherein the guard further comprises a window extending along the guard, and wherein the syringe further comprises a tab configured to travel within the window.
 5. The injection device of claim 4, wherein the window has a proximal edge and a distal edge, and wherein the tab abuts the proximal edge of the window when the guard is in the second position, the tab and proximal edge at least partially comprising the second cooperating detents.
 6. The injection device of claim 5, wherein the tab and proximal edge prevent the guard from advancing distally beyond the second position.
 7. The injection device of claim 5, wherein the tab and distal edge prevent the guard from moving proximally beyond the first position.
 8. The injection device of claim 1, wherein the first cooperating detents comprise a ledge on the proximal end of the syringe and a catch on the latch member, and wherein the guard is retained in the first position when the catch engages the ledge.
 9. The injection device of claim 8, wherein the latch member further comprises a proximal end for engaging the plunger, and a detent is located distally from the proximal end of the latch member.
 10. The injection device of claim 1, wherein the second cooperating detents comprise a detent and detent pocket on the guard and the syringe for preventing the guard from being moved from the second position toward the first position.
 11. The injection device of claim 1, wherein the guard further comprises a set of finger flanges for controlling movement of the guard from the first position toward the second position.
 12. An injection device, comprising: a syringe having a proximal end, a distal end for receiving a needle, and a plunger extending from the proximal end, the syringe having distal pockets; a guard slidably mounted to the syringe, the guard having a proximal end and a distal end, the guard being slidable from a first fixed position wherein a needle is exposed toward a second fixed position wherein the guard extends distally beyond the needle; a latch member extending proximally from the guard and coupled with the syringe to retain the guard in the first fixed position, the latch member being engageable by the plunger as the plunger is depressed for decoupling the latch member from the syringe, whereby the guard may be advanced toward the second position; and cooperating detents on the guard for engaging the pockets on the syringe for securing the guard in the second fixed position when the guard advances to the second position.
 13. The device of claim 12, further comprising a needle extending from the distal end of the syringe.
 14. The device of claim 12, further comprising a hub on the distal end of the syringe, and a needle coupled to the hub.
 15. The device of claim 12, wherein the guard is biased to move from the first position towards the second position.
 16. The device of claim 12, wherein the latch member comprises an intermediate portion for engaging a flange on the syringe and a proximal portion for engaging the plunger as the plunger is advanced into the syringe for disengaging the intermediate portion from the flange.
 17. The device of claim 12, wherein the guard further comprises a set of finger flanges disposed on opposite walls of the guard.
 18. An injection device, comprising: a syringe comprising a proximal end, a distal end, and a plunger extending from the proximal end, the syringe having distal pockets; a guard slidably attached to the syringe and having a proximal end and a distal end, the guard being biased from a first position wherein the distal end of the syringe is exposed toward a second position wherein the guard covers the distal end of the syringe; cooperating detents on the syringe for engaging the pockets on the guard for retaining the guard in the first position, and for retaining the guard in the second position; and a latch member extending proximally from the guard, the latch member being releasable by action of the plunger as the plunger is moved with respect to the syringe for releasing the first cooperating detents whereupon the guard may slide toward the second position.
 19. An injection device, comprising: a syringe having a proximal end, a distal end for receiving a needle, and a plunger extending from the proximal end, the syringe having distal pockets; a guard slidably mounted to the syringe, the guard having a proximal end and a distal end, the guard being slidable from a first fixed position wherein a needle is exposed toward a second fixed position wherein the guard extends distally beyond the needle; a latch member extending proximally from the guard and coupled with the syringe to retain the guard in the first fixed position, the latch member being releasable by action of the plunger as the plunger is moved with respect to the syringe, whereby the guard may be advanced toward the second position; and cooperating detents on the guard for engaging the pockets on the syringe for securing the guard in the second fixed position when the guard advances to the second position. 